The present invention relates generally to cryogenic cooling systems and more specifically the invention pertains to a system for the rapid temperature adjustment of samples between cryogenic temperatures and room temperatures.
The simplest way to cool something to liquid nitrogen temperature is to immerse it in liquid nitrogen. However, doing so usually allows impurities to cling to the object immersed, since liquid nitrogen generally is not completely pure. In addition, warming the sample from liquid nitrogen to room temperature in room air allows water to condense and/or freeze on the surface of the sample. For samples sensitive to water damage, such as high-temperature superconductors, this option is unattractive. In addition, simply encasing the object in some kind of rubber or latex sheath provides a non-durable, difficult to implement, and unreliable solution.
The task of adjusting the temperatures of samples between cryogenic and room temperatures is alleviated, to some extent, by the systems disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,385,010 issued to Horn;
U.S. Pat. No. 5,373,701 issued to Siefering;
U.S. Pat. No. 5,361,588 issued to Asami;
U.S. Pat. No. 5,357,758 issued to Andonian;
U.S. Pat. No. 5,355,456 issued to Osofsky;
U.S. Pat. No. 5,346,570 issued to Warden;
U.S. Pat. No. 5,275,016 issued to Chatlerjee; and
U.S. Pat. No. 5,241,828 issued to Kapitulnik.
There are many different types of dewars for cooling samples to liquid nitrogen temperatures that keep the sample dry. However, they can take hours to cool down, and cost thousands of dollars to purchase. Also, they can require large amounts of liquid nitrogen to operate compared with the invention described here. They usually use one of two methods for keeping the sample clear of liquid coolant and water: vapor flow, and vacuum chamber. A vapor flow system allows the coolant (liquid nitrogen or helium) to evaporate and blows the vapor across the sample. This increases the time required to cool down the sample and the complexity of the mechanism. The other method is placing the sample in a vacuum chamber, in contact with a piece of metal or other conductor of heat which is in turn in contact with the coolant. This method keeps all contaminating substances (like moisture-laden air and the liquid coolant) away from the sample, but also increases the time to cool the sample and the complexity of the device. Vacuums are difficult to obtain and maintain in a cryogenic environment if the seals are placed at cryogenic temperatures. Therefore, sample chambers often have to have the vacuum seal placed above the coolant level, which increases the size of the sample chamber and the difficulty of changing the sample. If the vacuum seal is placed below the coolant level, it requires a seal made of indium, which maintains its integrity at cryogenic temperatures, but is difficult to open and close.